The present invention relates to a talker speech level control circuit for a piezoelectric conversion telephone transmitter, capable of making spoken words clear and suppressing unnatural speech quality.
In order to ensure the articulation of speech to be transmitted from a telephone set, it is essential, in general, to consider environmental noises. To ensure the articulation of talker speech, it is necessary to suppress both low and high level signals.
Consider first low level signals. If a voice signal level is lower than a normal speech level and there are environmental noises, the ratio of environmental noises to a voice signal becomes large, to thereby degrading the articulation of spoken words. In such a case, it is desired to suppress the gain for both the environmental noises and the low level voice signals. Namely, if the sensitivity of low level voice signals relative to environmental noises is lowered, the environmental noises are apparently suppressed.
From the viewpoint of the conversion characteristic of a transmitter and limitation of signal level to be fed to a transmission line, it becomes also necessary to suppress the level of high sound pressure signal.
As described above, the talker's speech output is subjected to a compression process with respect to high and low level sound signals to thereby ensure the articulation of talker speech signals.
FIG. 2 is a circuit diagram showing a conventional general telephone set. In FIG. 2, reference numeral 1 represents a telephone line, 2 a handset, 3 a transmitter, and 4 a receiver. An output signal from the transmitter 3 is transmitted via a coupling capacitor 5, transmitter amplifier 6 and hybrid circuit 7 to the telephone line 1. A signal received from the telephone line 1 is supplied via the hybrid circuit 7, receiver amplifier 8 and coupling capacitor 9 to the receiver 4. The hybrid circuit 1 contains therein a talker speech output circuit having a function to regulate the talker signal level to match the impedance of the telephone line 1, and a sidetone attenuation circuit for attenuating an excessively large level sidetone.
There is known a carbon type acoustic-electric conversion element which converts an acoustic signal into an electric signal. FIG. 3 shows an example of the acoustic output characteristic of a carbon type acoustic-electric conversion element relative to an acoustic input signal level. The acoustic output characteristic shown in FIG. 3 indicates that the acoustic outputs for relatively low and high level (sound pressure) input signals are compressed.
More specifically, if a signal in excess of a certain level (sound pressure) is inputted to a carbon type transmitter, the output signal is compressed. A voice signal to be outputted to the telephone line is arranged to be suppressed to the order of 0 dBm (1 mW). The reason for this is that the sufficient dynamic range characteristics of the exchange and repeater at the succeeding stage can be obtained while retaining cost effectiveness, and that the sidetone level can be suppressed not to become too large and the speech level can be ensured while suppressing unnatural speech quality to the extent sufficient for practical use. From this viewpoint, a carbon type transmitter has a proper characteristic of compressing too a large input signal.
In addition, if a signal lower than a certain level (sound pressure) is inputted to a carbon type transmitter, the output signal is compressed. Therefore, the sensitivity to environmental noises is lowered to thus make conversation clear.
As described above, a carbon type transmitter itself has the characteristic that the acoustic outputs for relatively low and high level (sound pressure) input signals are compressed. It is necessary however to adjust the compression characteristic so as to obtain a desired characteristic.
Besides the carbon type transmitter using carbon as an acoustic-electric conversion element, a dynamic type, a ceramic type, an electret type or other type transmitters are also used in some cases.
The acoustic-electric conversion element using other than carbon has a more linear acoustic-electric conversion characteristic than that of the carbon acoustic-electric conversion element. Therefore, in order to ensure the articulation of speech, it is necessary to compress a converted electric signal greater than the carbon type transmitter.
As the means for compressing a high level sound pressure signal, there is a clip circuit (CL) for clipping at a predetermined level the electric signal converted from a talker speech at the hybrid circuit 7 shown in FIG. 2. The clip circuit is constructed of clamp diodes for example. If a voice signal to be transmitted becomes too large, the clamp diodes become conductive to accordingly perform a clip operation. At this time, the impedance of a balancing circuit (not shown) constituting the sidetone attenuator shown in FIG. 2 becomes unbalanced so that the sidetone attenuation operation becomes abnormal to thereby degrade the clearness of voices. Thus, while compressing a high level sound pressure signal, the impedance matching with the telephone line is lost, causing unnatural speech quality.
As described above, in a conventional telephone set, the compression process for an electric signal converted by the acoustic-electric conversion element is carried out in accordance with the characteristic specific to the conversion element. In this case, however, the sidetone attenuation effects are degraded or other problems occur such as unclearness of speech depending upon the voice level.